Temperature control for electric blankets



MW m3 i955 J. M. MQRAN -:TAL

TEMPERATURE CONTROL FOR ELECTRIC BLNKETS Filed Jan. 3, 1952 hui TEMPERATURE CNTRL FOR ELECTRIC BLANKETS James M. Moran, Arlington, and Horace K. MacKechnie, Lexington, Mass., assignors, hy mesue assignments, to Knapp-Monarch Company, St. Louis, Mo., a corporation of Delaware Application January 3, 1952, Serial No. 264,772

12 Claims. (Cl. 219--46) This invention relates to a temperature control for electric blankets.

The object ot the invention is to provide a novel and improved temperature control for electric blankets having provision for manually adjusting the temperature at which the heat is discontinued in the blanket and wherein provision is further made for automatically compensating for ambient temperature variations in a novel and superior manner.

With this general object in View, and such others as may hereinafter appear, the invention consists in the temperature control hereinafter described and particularly defined in the claims at the end of this specification.

ln the drawing illustrating the preferred form of the invention, the single ligure is a diagrammatic View of the circuit arrangement for the present novel temperature control.

in general, the present invention contemplates a novelV temperature control for an electric blanket wherein the heating circuit includes a normally balanced dual coil relay having opposed windings including a series coil and a shunt or parallel coil arranged so that the magnetic forces oppose each other and wherein an increase in the resistance of the blanket heating element, upon heating thereof, reduces the current in the series coil so that the shunt or parallel coil overbalances the series coil and causes suiicient magnetization of the core of the relay to operate a normally closed switch to discontinue the current in the blanket heating element. Upon opening or' the normally closed relay switch, provision is made for actuating a normally closed thermal time-delay switch arranged to be operated by a heater which is energized when the relay switch is opened so that after a predetermined time the thermal switch is opened to discontinue the current in the entire heating circuit, whereupon the relay switch is again closed in readiness for a succeeding cycle upon cooling and closing of the thermal switch.

Provision is also made for manually varying the point at which the two coils become unbalanced, as by a rheostat placed in the shunt circuit, so as to change the temperature at which the blanket heating element will be cut off, and in order to compensate for ambient temperature, a heat-responsive resistor having a negative coeicient ot resistance is also provided in the shunt circuit so that when the resistor element is cool the resistance therethrough is relatively high, and as the temperature increases the resistance decreases, thereby automatically changing the current flow in the shunt circuit with the ambient temperature.

Referring now to the wiring diagram shown in the drawing, represents a blanket, indicated in dotted lines, and 12 represents the heating element associated with the blanket. The heating element 12 is connected in a circuit including wires 14, 16 extended froma conventional electric-service plug 18, the wire 16 being connected directly to one terminal 2li of the blanket heating element 12. The wire 14 is connected to a maincontrol switch 22 for closing the circuit through a wire 24 which ice 2 is connected to a thermal time-delay switch, indicated generally at 25, forming a part of the control mechanism. The switch 25 is provided with contacts 26, 28 and a condenser 3G may be connected between the terminals thereof, as shown.

A wire 32 leading from the time-delay unit is connected to a dual control relay, indicated generally at 35, having a core 34 and provided with opposed windings including a series winding 36 and a shunt or parallel winding 53 arranged so that the magnetic forces oppose each other. rthe v ire 32 is connected to one terminal 40 of the series winding 36 and the other terminal 42 is connected by a wire 4d to a contact 46 carried by the armature of the dual control relay. The contact 46 norly engages a contact 47 having a wire 5) extended therefrom and connected to the other terminal 52 of the blanket heating element 12 through a fuse S4. A condenser is placed across the contacts to prevent arcing of the points. When the relay is energized the pivoted armature 48 is rocked to open the circuit at the contacts 46, 47. A time-delay unit heater 56 arranged for cooperation with the thermal time-delay switch 2S is provided in a line 5S placed across the contacts of the relay so as to be energized when the circuit is opened at the contacts 46, 47.

rhe shunt circuit includes a wire 6i) extended from the wire 32 and connected to one terminal 62 of the shunt winding 3%, the second terminal 64 being connected to the wire io through a heat-sensitive resistor rheostat 'itl and wire 72. l"the heat-sensitive resistor 63 has a negative coeiiicient of resistance so that when cool the resistance therethrough is relatively high, and as the temperature increases the resistance decreases, thereby automatically changing the current llow through the shunt winding 33. A suitable resistance 69 is placed across the resistor 6b in order to limit the maximum resistance value at low ambient temperatures. This is desirable to permit starting of the blanket circuit at low ambient temperatures. A signal lamp 74 is provided in a line '76 extended between the wires 24, 16 and provided with a resistance 73, the lamp being lighted when the main-control switch 22 is closed. ln operation, when the main-control switch 22 is closed, current may ow to the blanket heating element 12, and the blanket ternperature may be set the user by adjusting the rheostat "ill to vary the current the shunt winding 38. A higher resistance reduces the eiect of the shunt winding 3S relative to the series winding 36 giving a higher blanket temperature, and a lower resistance to ampere flow increases the magnetic effect of the shunt winding 33 giving a cooler temperature. in practice, the rheostat 70 may be calibrated for different temperatures orl the blanket heating element.

The two windings 36, 38 of the relay 35 are arranged in approximate balance over the operating ranges selected so that when the blanket is cool the coiis produce a minimum magnetization in the core 34 oi the relay. When the blanket heating ciement 12 becomes heated, the resistance increases, thus reducing the current in the series winding 3 so that the shunt coil 38 overbalances the series coil 36, and when the imbalance reaches the ampere turns required to operate t`.e relay 35S the normally closed relay contacts 47 are opened to discontinue heating of the blanket.

@peiling or the relay contacts i7 causes 'the current to ilow through the heater unit 56 of the thermal timedelay switch 2S which after a predetermined time effects separation of the thermal contacts lo, 2S to open-circuit the entire unit. @peiling ol the circuit at the thermal switch 25 allows the relay contacts 46, 457 to close ag n for the next cycle. When the thermal Vtime-delay unit has cooled sufficiently, its contacts 26, again close and the heating cycle is repeated. If the average temperature of the blanket has dropped, the series relay circuit will remain closed until the blanket is again up to the predetermined selected temperature. If the temperature of the blanket is the same as when the heat was previously shut olf, the relay contacts will immediately reopen and recycle through the time-delay units, this action being repeated until the blanket has cooled slightly whereupon the relay contacts 46, 47 will remain closed, as above described. The thermo-fuse 5d is preferably mounted very close to the relay contacts d6, 47 and has two ir portant functions. in case of overload or short circuit in the blanket it opens the circuit. Defective circuitry or components which cause the relay contacts to overheat will blow this fuse Sd instead of fusing the relay contacts.

Temperature compensation or automatic increase in blanket temperature with lowering ambient is accomplished by the temperature characteristic of the shunt resistor 68 and to some extent by the heat-transfer characteristics of the relay coils and the design of the thermal time-delay unit. The temperature-responsive resistor 68, having a negative coeiiicient of resistance, operates automatically to increase the resistance through the shunt winding 3i; when the temperature ambient is lowered, thus shifting the balance point of the windings 36, 3S so that with lowering ambient the relay contacts will remain closed for' a longer time before the unbalanced point is reached to effect operation of the relay and discontinuance of the current to the blanket heating element. Conversely, when the temperature ambient increases, the resistance through the shunt winding 3S decreases, thereby changing the current dow in proportion to the temperature ambient change.

The resistance 68 also serves, in addition to temperature compensation, as a means for correcting blanket temperature variations due to line voltage fluctuations. if the line voltage is low the current has to be applied to the blanket for a longer time to obtain a given blanket temperatur-e. in other words, as the circuit goes on and oii .it spends more on-time. The balance characteristic of the relay is such that the blanket temperature tends to increase with decreasing line voltage. However, due to thc longer on-time resistance 63 heats up more which reduces the ctective resistance and shuts oit the blanket sooner. The opposite is true of higher line voltage.

From the above description, it will be seen that the present electric-blanket control permits the user to adiust the rheostat 7i) to the desired setting to eiect discontinuance of the current to the blanket heating element a.

12 at a predetermined safety cut-oli temperature, and that upon lowering ambient the resistance through the shunt winding 38 is automatically changed by the provision of the temperature-responsive resistor 68 to compensate for lowering ambient temperature. It will also be seen that the thermal time-delay unit is operated only when the reiay contacts 46. 47 are opened, and that the present control unit employs the relay as the temperature-control component as well as a safety device.

While the preferred embodiment of the invention has been herein illustrated and described, it will be understood that the present invention may be embodied in other forms within the scope of the following claims.

Having thus described the invention, what is claimed is:

l. in a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coetiicient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said diferential relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to effect opening of the relay contacts upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, and a heat-responsive resistor connected in series with said shunt winding and having a negative temperature coefficient of resistance, said resistor being positioned outside the blanket and suiliciently spaced therefrom to be exposed to ambient temperature so as to automatically vary said predetermined temperature in response to variations in ambient temperature.

2. In a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coefcient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said dilerentiai relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parailel with said heating element, said windbeing arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to eiect opening ot' the relay contacts upon an increase in resistance ot said heating element due to heating of the blanket to a predetermined temperature,` a manually adjustable rhec-stat in series with said shunt winding to provide for manually varying said predetermined temperature, and a heat-responsive resistor connected in series with said shunt winding and having a negative tem erature coeicient of resistance, said resistor being positioned outside the blanket and sufficiently spaced therefrom to be exposed to ambient temperature so as to automatically vary said predetermined temperature in response to variations in ambient temperature.

3. In a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeicient of resistance, a differential relay having normally closed contacts in said circuit in series with said heating element and a said differential relay comprising a series winding in sad circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to eiect opening of the relay contacts and end a heating cycle upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, a thermal time-delay switch in series with both of said windings, and a heater connected across the contacts of the relay cooperating with the thermal switch and energized upon opening of the relay contacts, said heater upon being energized heating the thermal switch and effecting opening of the thermal switch after a predetermined time, said relay contacts again closing upon opening of the thermal switch to condition said heating circuit for another heating cycle upon subsequent cooling and closing of the thermal switch, and a heat-responsive resistor connected in series with said shunt winding and having a negative temperature coefficient of resistance, said resistor being positioned outside the blanket and suiciently spaced therefrom to be exposed to ambient temperature so as to automatically vary said predetermined temperature in response to variations in ambient temperature.

4. In a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeflicient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said differential relay comprising a series winding in said circuit in series with said heatng element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to effect opening of the relay contacts and end a heating cycle upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, a thermal time-delay switch in series with both of said windings, and a heater connected across the contacts of the relay cooperating with the thermal switch and energized upon opening of the relay contacts, said heater upon being energized heating the thermal switch and effecting opening of the thermal switch after a predetermined time, said relay contacts again closing upon opening of the thermal switch to condition said heating circuit for another heating cycle upon subsequent cooling and closing of the thermal switch, and. a manually adjustable rheostat in series with said shunt winding to provide for manually varying said predetermined temperature, and a heatresponsive resistor connected in series with said shunt winding and having a negative temperature coeicient of resistance, said resistor being positioned outside the blanket and suliiciently spaced therefrom to be exposed to ambient temperature so as to automatically vary said predetermined temperature in response to variations in am ient temperature.

5. .in a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeicient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said differential relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to effect opening of the relay contacts upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, and a heat-responsive resistor connected in series with said shunt winding and having a negative temperature coehcient of resistance, said resistor being positioned outside the blanket and suiiiciently spaced therefrom to be exposed to ambient temperature so as to automatically vary said predetermined temperature in response to variations in ambient temperature, and resistor means connected in parallel with said heatresponsive resistor for limiting the maximum value of resulting resistance of said heat-responsive resistor and said resistor means at low ambient temperature in order to prevent immediate opening of the relay contacts when initially starting operation of the blanket at said low ambient temperature.

6. ln a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeiiicient or" resistance, a diterential relay having normally closed contacts in said circuit in series with the blanket heating element, said differential relay comprising a series winding in said circuit in series with said heatng element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to effect opening of the relay contacts and end a heating cycle upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, a thermal timedelay switch in series with both of said windings, and a heater connected across the contacts of the relay cooperating with the thermal switch and energized upon opening of the relay contacts, said heater upon being energized heating the thermal switch and eiecting opening of the thermal switch after a predetermined time, said relay contacts again closing upon opening of the thermal switch to condition said heating circuit for another heating cycle upon subsequent cooling and closing of the thermal switch, a manually adjustable rheostat in series with said shunt winding to provide for manually varying said predetermined temperature, and a heatresponsive resistor connected in series with said shunt winding and having a negative temperature coeiicient of. resistance, said resistor being positioned outside the blanket and suilicient spaced therefrom to be exposed to ambient temperature so as to automatically vary said predetermined temperature in response to variations in ambient temperature, and resistor means connected in parallel with said heat-responsive resistor for limiting the maximum value of resulting resistance of said heat-responsive resistor and said resistor means at low ambient temperature in order to prevent immediate opening of the relay contacts when initially starting operation of the blanket at said low ambient temperature.

7. in a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeliicient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said differential relay comprising a series winding in said circuit in series with sad heating clement and a shunt winding connected in paraliel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to eifect opening ot the relay contacts upon an increase in resistance ot said heating element due to heating of the blanket to a predetermined temperature, and a heat-responsive resistor connected in series with said shunt winding and having a negative temperature coeilicient of resistance, said resistor being positioned outside the blanket and spaced therefrom.

8. ln a temperature control for an electric blanket, a heating circuit including blanket heating element having a positive temperature coeficient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said differential relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to effect opening of the relay contacts upon an increase in resistance o'r` said heating element due to heating of the blanket to a predetermined temperature, a manually adjustable rheostat in series with said shunt winding to provide for manually varying said predetermined temperature, and a heat-responsive resistor connected in series with said shunt winding and having a negative temperature coeliicient of resistance, said resistor being positioned outside the blanket and spaced therefrom.

9. ln a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coefficient of resistance, a diferential relay having normaliy closed contacts in said circuit in series with the blanket heating element, said diferential relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to eiiect opening of the relay contacts and end a heating cycle upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, a thermal time-delay switch in series with both of said windings, and a heater connected across the contacts of the relay cooperating with the thermal switch and energized upon opening of the relay contacts, said heater upon being energized heating the thermal switch and eiecting opening of the thermal switch after a predetermined time, said relay contacts again closing upon opening of the thermal switch to condition said heating circuit for another heating cycie upon subsequent cooling and closing of the thermal switch, and a heatresponsive resistor connected in series with said shunt winding and having a negative temperature coeiicient of resistance, said resistor being positioned outside the blanket and spaced therefrom.

10. In a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeicient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said differential relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to Aeiect opening of the relay contacts and end a heating cycle upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, a thermal time-delay switch in series with both of said windings, and a heater connected across the contacts of the relay cooperating with the thermal switch and energized upon opening or the relay contacts, said heater upon being energized heating the thermal switch and effecting opening of the thermal switch after a predetermined time, said relay contacts again closing upon opening of the thermal switch to condition said heating circuit for another heating cycle upon subsequent cooling and closing of the thermal switch, and a manually adjustable rheostat in series with said shunt winding to provide for manually varying said predetermined temperature, and

a heat-responsive resistor connected in series with said shunt winding and having a negative temperature coeflcient of resistance, said resistor being positioned outside the blanket and spaced therefrom.

11. In a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeicient of resistance, a differential relay having normally closed contacts in said circuit in series with the blanket heating element, said dilerential relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to effect opening of the relay contacts upon an increase in resistance of said heating element due to heating of the blanket to a predetermined temperature, and a heat-responsive resistor connected in series with said shunt winding and having a negative temperature coefiicient of resistance, said resistor being positioned outside the blanket and spaced therefrom, and resistor means connected in parallel with said heat-responsive resistor.

12. In a temperature control for an electric blanket, a heating circuit including a blanket heating element having a positive temperature coeicient of resistance, a dilerential relay having normally closed contacts in said circuit in series with the blanket heating element, said differential relay comprising a series winding in said circuit in series with said heating element and a shunt winding connected in parallel with said heating element, said windings being arranged in opposed and normally balanced relation for controlling the energization of the blanket heating element, said windings assuming an unbalanced relation to eiiect opening of the relay contacts and end a heating cycle upon an increase in resistance of said heating element due to heating of theblanket to a predetermined temperature, a thermal time-delay switch in series with both of said windings, and a heater connected across the contacts of the relay cooperating with the thermal switch and energized upon opening of the relay contacts, said heater upon being energized heating the thermal switch and eecting opening of the thermal switch after a predetermined time, said relay contacts again closing upon opening of the thermal switch to condition said heating circuit for another heating cycle upon subsequent cooling and closing of the thermal switch, a manually adjustable rheostat in series with said shunt winding to provide for manually varying said predetermined ternperature, and a heat-responsive resister connected in series with said shunt Winding and having a negative temperature coefficient of resistance, said resistor being positioned outside the blanket and spaced therefrom, and resistor means connected in parallel with said heat-responsive resistor.

Retieiences Cited in the lle of this patent UNTED STATES PATENTS 919,402 Trumpler Apr. 27, 1909 1,183,814 Haagn May 16, 1916 1,528,053 Hands Mar. 3, 1925 2,057,902 Moreau Oct. 20, 1936 2,477,819 Newell Aug. 2, 1949 2,549,095 Huck Apr. 17, 1951 2,592,525 Huck Apr. 15, 1952 OTHER REFERENCES McNairy: Abstract of Appl. No. 725,852, pub. April 11, 1950. 

